Digital alchemy
"Its terminal emulator can't handle modern apps such as htop or the Tilde text editor".
This is a problem of lack of understanding of terminals and terminal emulators by the system administrator. Similar problems are faced by using Putty or terminals on Linux systems to access proper legacy UNIX systems.
The issue is that not all terminal emulators are vt220, xterm et. al. compatible, and in fact xterm is not a very safe setting for the TERM environment variable used to condition the terminfo entries, as there have been just soooo many mostly compatible, but ultimately not the same versions of "xterm" across the years.
I believe that the correct setting for TERM with the CDE terminal emulator should be "dtterm", but I would suspect that many Linux systems do not have a dtterm terminfo entry, so fall back to xterm, or xterm-256color or something similar. This will almost certainly not match the capabilities of the dtterm terminal emulation.
The common problems are:
Function keys not being recognized correctly
Non-7-bit-ascii characters do not work correctly, especially box draw characters
Any colour support will be very spotty
Some cursor movement operations do not work correctly.
Many of these problems can be fixed at one fell swoop, by identifying the location of the dtterm terminfo file, and making sure there is a copy in the appropriate place for the hosting OS that you are using (unless, like me, you add to the terminfo database with local additions).
The one that will possibly cause a problem is the font that is used, as I'm pretty certain that you will have to have iso8859 fonts in your font path, rather than just UTF-8 ones, unless the version of dtterm on NsCDE has been altered for UTF.
I've just fired up an AIX 5.3 system, and installed an original version of CDE on it, and then run a dtterm via X11 onto a RHEL 8.6 system, and things work pretty much OK, although I don't have "-dt-interface user-medium-r-normal-m" at any size in my default font path.
You cannot imagine how frequently I find I want to take a UNIX or Linux administrator who accepts the wrong characters for box draw or unrecognized function keys as something normal, and try to shake some knowledge into them, because it is nearly always user error, not a problem with the system! UNIX was written to allow *LOTS* of different terminals types to use the system correctly, and Linux mostly inherited the capabilities.
Things have become both more simple, while at the same time more complex with the commodification of UNIX-like OSs, such that what was once well known appears almost like alchemy nowadays.